1. Field of the Invention
This invention relates to a polyethylene film for packaging photosensitive materials for photographic purposes, particularly a laminated film containing a layer of linear low density polyethylene resin manufactured by a low pressure method.
2. Description of Prior Arts
Polyethylene film is widely employed as a packaging material. Most of the polyethylene films are made of low density polyethylene manufactured by a high pressure method or high density polyethylene manufactured by a low pressure method. Recently, linear low density polyethylene manufactured by low pressure methods (L-LDPE) has been developed. This L-LDPE has both advantages of the polyethylene of the high pressure method and the polyethylene of the low pressure method, and it is noticed because of its low cost and high strength which are in accord with current requirements, i.e. to save energy and to save resources. However, since transparency of L-LDPE is insufficient, it is difficult to replace the low density polyethylene for the use in general packaging by the L-LDPE. Moreover, since processing character of the L-LDPE are inferior, it is also difficult to replace in the field where transparency is not required.
As the packaging materials for photosensitive materials for photographic purposes, it is necessary that they possess various properties such as light-shielding, gas barrier, moisture proofing, physical strengths such as breaking strength, tear strength, impact puncture strength and Gelbo test strength, heat sealing properties such as heat seal strength and hot tack properties (hot-seal ability) and antistatic properties and the like. Generally, it is difficult to satisfy these properties by a single layer film, and accordingly, a laminated film is usually employed. Such a laminated film is, for example, composed of a low density polyethylene film containing carbon black or a pigment and a flexible sheet such as paper, aluminum foil or cellophane.
An example of conventional films is shown in FIG. 13. This film is a laminated film composed of a film of a low density polyethylene manufactured by a high pressure method containing carbon black, an aluminum foil and a flexible sheet.
On the other hand, the present inventor has already developed a film for packaging photosensitive materials for photographic purpose using L-LDPE (Japanese Patent Application Kokai No. 58-132555). This film is characterized by containing at least one layer of polyethylene polymer of which more than 50 wt% is L-LDPE and which contains more than 1 wt% of a light-shielding material.
Physical properties of the conventional laminated films are not enough, and during packaging, the films were sometimes torn, punctured or the heat seal of the films sometimes separated. In addition, when a large amount of a light-shielding material such as carbon black is added, physical strength of the film is lowered. Then, the amount was set about 3%, and thickness of the film was more than 70 .mu.m. As the result, the film was stiff, and working efficiency of the packaging process was badly influenced. Cost of the packaging was also expensive.
In the case of the film using L-LDPE described previously, physical strength was improved. However, when aluminum foil was laminated in order to impart gas barrier, to raise moisture proofing, to impart antistatic properties, etc., tear strength of the film fell less than one half in spite of the increased thickness.